A kettle sour beer is soured using bacteria in the brewing kettle before the main fermentation process begins. This method allows brewers to control the level of sourness more precisely and offers much greater efficiency compared to traditional methods of souring beer by aging it in wood with a mix of microbes, and, most of all, time. Traditional sours can take several months or even years to create, while a kettle sour can be turned around in a matter of days.

Kettle souring typically involves introducing lactobacillus bacteria to the wort (unfermented beer) and allowing it to sour in the kettle overnight or for a couple of days. After the desired level of sourness is achieved, the wort is boiled to kill the bacteria before yeast is added for fermentation. This process results in a tart and refreshing beer style that has become increasingly popular among craft brewers.